1. Field of the Invention
The present invention relates generally to passive optical networks (PONs) and, more specifically, to troubleshooting a PON using an Optical Time-Domain Reflectometer (OTDR) or other optical instrument.
2. Description of the Related Art
Most digital telecommunications networks (i.e., networks that facilitate the communication of data, voice, video, etc., between parties or between a content distribution service and subscribers) typically comprise active components, such as repeaters, relays and other such devices that consume power, in the path between a central office (or exchange, as its sometimes referred to) and a subscriber. In addition to requiring power, active components are subject to failure and performance degradation over time, and may require significant periodic maintenance. The passive optical network (PON) has been developed to overcome some of these deficiencies. The essence of a PON is that nothing but optical fiber and passive components are found in the path between the central office and subscribers. A single fiber can run from the central office to a passive splitter located near a group of subscribers, such as a neighborhood or office complex, and individual fibers can run from the splitter to individual subscribers or sub-groups of subscribers. Splitters can be cascaded to reach a greater number of subscribers.
The International Telecommunications Union (ITU) and the Institute of Electrical and Electronics Engineers (IEEE) are two standards-making bodies currently developing PON standards. The ITU has adopted recommendations of the Full Service Access Networks (FSAN) organization, including G983.x, a specification sometimes referred to as “broadband PON” (BPON), and G984.x, a specification sometimes referred to as “gigabit PON” (GPON). The IEEE has also adopted IEEE 802.3-based PON standards referred to as “Ethernet PON” (EPON) and “gigabit EPON” (GEPON). These standards and recommendations are well-known to persons skilled in the art to which the invention relates and are therefore not described in further detail in this patent specification (“herein”).
In accordance with these standards, a PON comprises an optical line termination (OLT), which is typically located at the central office, and a number of optical network termination (ONTs) (also known as optical network units), each located at the subscriber's premises (e.g., home, office building, etc.), with optical fiber and one or more splitters between the OLT and ONTs. In the downstream direction, i.e., data transmitted from the OLT (e.g., located at the central office) to an ONT (e.g., located at a subscriber's premises), the data units are broadcast from the OLT to all of the ONTs on the PON, and an ONT can select the data to receive by matching the address embedded in the data units to a previously provisioned or learned address. In other words, an ONT only “listens” to data units having a matching address. Thus, the OLT can transmit data to a particular or selected ONT by addressing it to that ONT. In the upstream direction, i.e., data transmitted from an ONT to the OLT, the data units are time-domain multiplexed.
An Optical Time-Domain Reflectometer (OTDR) is an instrument that is commonly used to analyze optical networks for troubleshooting or set-up purposes. An OTDR analyzes the light loss in an optical fiber by transmitting a (laser) light pulse into the optical fiber and measuring the backscatter and reflection of light as a function of time. The reflected light characteristics are analyzed to determine the location of any broken or damaged fibers, faulty connectors, splice losses, or other faults.
While an OTDR can successfully be used to troubleshoot many types of optical networks, signal losses from the passive splitters hamper its use in a typical PON of the type that provides video, voice, Internet service, etc., from a service provider to subscribers. As illustrated in FIG. 1, the conventional manner in which an OTDR 10 is used to troubleshoot a PON involves disconnecting the OLT 12 from the PON and temporarily substituting (indicated by dashed line) OTDR 10. The PON architecture shown in FIG. 1 is intended to be illustrative of a typical network. Note that each of the eight outputs of splitter 14 is coupled to the input of each of eight other 1:8 splitters 16. That is, splitters 16 are cascaded with splitter 14 to form a two-level cascaded arrangement or topology. The output of each of splitters 16 is coupled to an ONT 18. (The omission of some splitter outputs and ONTs 18 for purposes of clarity is indicated by an ellipsis (“ . . . ”) symbol.)
In use, light emitted by OTDR 10 travels through several spans of fiber as well as splitters 14 and 16 before reaching an ONT 18 and reflecting back through the same fiber and splitters 14 and 16 to OTDR 10. This arrangement does not work well because the fiber and splitters can contribute a total signal loss that can exceed the useful dynamic range of OTDR 10. For example, if the fiber contributes five decibels (dB) of loss, and each splitter contributes 10 dB, the total one-way loss is 25 dB, and the total two-way loss is 50 dB. It is generally not possible to take useful OTDR measurements where the signal loss is 50 dB. When OTDR measurements have been completed, OTDR 10 is disconnected from the PON and OLT 12 is re-connected.
Splitters having multiple inputs and multiple outputs are known in the art and have been used, for example, to couple a video overlay signal onto a PON in parallel with the OLT. In splitter nomenclature, a splitter having one input and eight outputs, for example, is commonly referred to as a “1:8” splitter, a splitter having two inputs and eight outputs, for example, is commonly referred to as a “2:8” splitter, etc.
It would be desirable to perform optical time-domain reflectometry in a PON in a manner that is not hampered by splitter and fiber losses. The present invention addresses these problems and deficiencies and others in the manner described below.